1. Field of the Invention
The present invention is directed to the technology of iron and steel manufacture and, more particularly, to an integrated mini-mill based on direct reduced iron and electric arc furnace steel making.
2. Description of Related Art
Today, integrated mini-mills for iron and steel making have separate units for iron making and for steel making. Almost all produce iron separately in reduction reactor(s), cool and store the iron, and then use the iron in the steel making unit. Several methods have been developed to integrate the iron and steel making units. However, these concepts have failed to successfully and economically integrate the two systems. The present concepts are plagued by high investment and operating costs and ever rising energy costs. No system has been created which efficiently transfers the iron from the iron-making unit to the steel-making unit and efficiently utilizes energy sources.
As direct reduction reactor (DRR) diameters become larger, the present system of charging iron ore into vertical shaft DRR used by Midrex and HYL will become impracticable because of:
i) the extreme heights to which the structures must be raised,
ii) poor distribution of the raw materials inside the reactor with the fine ores tending to segregate from the coarser ores and causing channeling of gases, and
iii) limitations in top gas pressures.
Several methods have been developed for transporting direct reduced iron (DRI) to an electric arc furnace (EAF), for example, refractory lined containers, specially designed trucks, pneumatic transportation systems, and high temperature metallic conveyors. These methods have numerous problems, such as logistics and coordination with other parts of the mill, heavy capital investment, considerable heat loss, excessive loss of metallization, and the need for intermediate storage.
Current technologies using pressure swing adsorbers or vacuum pressure swing adsorbers (PSA/VPSA), to treat top gas from the DRR, require that the top gas be cooled and cleaned of dust before the PSA/VPSA, and the reducing gas from the PSA/VPSA be again reheated to the required reduction temperatures prior to being re-introduced into the DRR. High levels of carbon monoxide (CO) in the gas cause problems of xe2x80x9cmetal dustingxe2x80x9d of the high temperature Nixe2x80x94Cr alloy reheater tubes. This leads to frequent failures of tubes, high maintenance costs, and plant stoppages.
It is, therefore, an object of this invention to avoid the above problem and others by providing a mini-mill for steel making which integrates the iron making unit with the steel making unit. Still other objects will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description.
Accordingly, we have invented an integrated iron and steel making unit that eliminates the transportation and handling of hot DRI over long distances. The goal is to deliver the hot DRI to the EAF with a minimum loss of metallization utilizing simple and proven equipment that is easy to operate and which has a low capital investment. Additionally, the integrated iron and steel making unit leads to improved productivity in steel making; maximum operating efficiency; lower power, electrode and refractory consumptions; shorter tap-to-tap times; smaller furnaces and transformers; and overall capital cost reduction.
A method and apparatus for iron making according to the present invention includes charging a DRR with iron ore from a charging system. The iron ore is reduced to hot DRI in the DRR. At the bottom section of the DRR, at least one screw feeder discharges the hot DRI to at least one rotary kiln. The rotary kiln transports the hot DRI to at least one EAF. Slag and liquid steel are produced by and periodically tapped from the EAF in a conventional manner.
Top gas (i.e., spent reducing gas) is drawn off of a top section of the DRR. The main portion of the top gas flows to a PSA/VPSA for CO2 and H2O removal. The gases exiting the PSA/VPSA are a reducing gas, comprised mostly of CO and H2, and a tail gas, comprised mostly of CO2 and H2O but with some useful calorific value still remaining. The tail gas from the PSA/VPSA can be used elsewhere in the plant as fuel. A small portion of the tail gas is used to pressurize the rotary kiln to prevent air from entering the rotary kiln and oxidizing the DRI.
At least one plasma torch may be used to reform natural gas, oxygen, and top gas from the DRR to form a hot reducing gas rich in CO and H2. The hot reducing gas is mixed with the cool reducing gas exiting the PSA/VPSA to form a final reducing gas. The final reducing gas is delivered to the DRR at the required reduction gas temperature.
The charging system of the present invention is akin to a blast furnace charging system which thereby eliminates the need for a tall charging system, as used with current DRRs. The lower height of the present invention decreases structural costs and accessibility to the equipment.
The use of a rotary kiln to transport the hot DRI directly from the DRR to the EAF eliminates the need to cool the DRI for transport and/or storage prior to charging the EAF as well as eliminating the need to subsequently reheat the DRI for the steel making unit. Considerable savings in energy of steel making are achieved.
The use of the PSA/VPSA to remove carbon dioxide and water from the top gas from the DRR and the recycling of the gas rich in CO and H2 results in the elimination of or reduction in the size of catalytic reformers over conventional catalytic reformers. Since adsorption requires no natural gas, natural gas consumption otherwise required to produce reducing gas in a catalytic reformer, is eliminated or reduced.
The use of a plasma torch generates more reducing gas and serves to preheat the gas from the PSA/VPSA. Mixing the hot and cool gases avoids the need to recycle the cool gas through a separate pre-heater or through the catalytic reformer. Problems of metal dusting of Nixe2x80x94Cr tubes normally associated with gases rich in CO will thereby be avoided.